Newly documented antimicrobial activity of quinolones

Quinolone: a versatile therapeutic compound class

The discovery of nalidixic acid is one pinnacle in medicinal chemistry, which opened a new area of research that has led to the discovery of several life-saving antimicrobial agents (generally referred to as fluoroquinolones) for over decades. Although fluoroquinolones are frequently encountered in the literature, the utility of quinolone compounds extends far beyond the applications of fluoroquinolones. Quinolone-based compounds have been reported for activity against malaria, tuberculosis, fungal and helminth infections, etc. Hence, the quinolone scaffold is of great interest to several researchers in diverse disciplines. This article highlights the versatility of the quinolone pharmacophore as a therapeutic agent beyond the fluoroquinolone profile.

Validation of Putative Apicoplast-Targeting Drugs Using a Chemical Supplementation Assay in Cultured Human Malaria Parasites

Malaria parasites contain a relict plastid, the apicoplast, which is considered an excellent drug target due to its bacterial-like ancestry. Numerous parasiticidals have been proposed to target the apicoplast, but few have had their actual targets substantiated. Isopentenyl pyrophosphate (IPP) production is the sole required function of the apicoplast in the blood stage of the parasite life cycle, and IPP supplementation rescues parasites from apicoplast-perturbing drugs. Hence, any drug that kills parasites when IPP is supplied in culture must have a nonapicoplast target. Here, we use IPP supplementation to discriminate whether 23 purported apicoplast-targeting drugs are on- or off-target. We demonstrate that a prokaryotic DNA replication inhibitor (ciprofloxacin), several prokaryotic translation inhibitors (chloramphenicol, doxycycline, tetracycline, clindamycin, azithromycin, erythromycin, and clarithromycin), a tRNA synthase inhibitor (mupirocin), and two IPP synthesis pathway inhibitors (fosmidomycin and FR900098) have apicoplast targets. Intriguingly, fosmidomycin and FR900098 leave the apicoplast intact, whereas the others eventually result in apicoplast loss. Actinonin, an inhibitor of bacterial posttranslational modification, does not produce a typical delayed-death response but is rescued with IPP, thereby confirming its apicoplast target. Parasites treated with putative apicoplast fatty acid pathway inhibitors could not be rescued, demonstrating that these drugs have their primary targets outside the apicoplast, which agrees with the dispensability of the apicoplast fatty acid synthesis pathways in the blood stage of malaria parasites. IPP supplementation provides a simple test of whether a compound has a target in the apicoplast and can be used to screen novel compounds for mode of action.

Antiviral, antifungal, and antiparasitic activities of fluoroquinolones optimized for treatment of bacterial infections: a puzzling paradox or a logical consequence of their mode of action?

This review summarizes evidence that commercially available fluoroquinolones used for the treatment of bacterial infections are active against other non-bacterial infectious agents as well. Any of these fluoroquinolones exerts, in parallel to its antibacterial action, antiviral, antifungal, and antiparasitic actions at clinically achievable concentrations. This broad range of anti-infective activities is due to one common mode of action, i.e., the inhibition of type II topoisomerases or inhibition of viral helicases, thus maintaining the selective toxicity of fluoroquinolones inhibiting microbial topoisomerases at low concentrations but mammalian topoisomerases at much higher concentrations. Evidence suggests that standard doses of the fluoroquinolones studied are clinically effective against viral and parasitic infections, whereas higher doses administered topically were active against Candida spp. causing ophthalmological infections. Well-designed clinical studies should be performed to substantiate these findings.

Efficacy of different treatment regimens of marbofloxacin in canine visceral leishmaniosis: a pilot study

This phase II, randomized, open-label field trial was designed to evaluate and compare the safety and efficacy of four treatment durations (10, 20, 28 or 40 days) with marbofloxacin administered orally at the dosage of 2mg/kg once a day for canine visceral leishmaniosis. Twenty-four dogs naturally infected with visceral leishmaniosis and without biochemical disorder evidences of renal insufficiency, were recruited by two Greek veterinarian clinics. They were also randomly assigned to one of the four treatment duration groups, and have been clinically, haematologically, biochemically and parasitologically followed-up regularly for 9 months. Efficacy was achieved for 5/6 dogs treated for 28 days, 4/6 dogs treated for 10 or 20 days and for 3/6 dogs treated for 40 days. Moreover, efficacy was reached more quickly (58.4 days) in dogs treated for 28 days. Improvement of clinical signs tended to be better and faster in the 28 days treatment group too. After 9 months of follow-up, a total of three cases could be considered as relapsing (two dogs treated for 40 days and one dog treated for 28 days). There was a significant reduction in amastigotes density in macrophages after 3 months in the four groups when compared with the parasite density at inclusion. No adverse effects were noticed during this 9 months study. Results obtained with marbofloxacin at the dosage of 2mg/kg once a day for 28 days seem encouraging and may offer a safe alternative for treating canine visceral leishmaniosis.

'FAS't inhibition of malaria

Malaria, a tropical disease caused by Plasmodium sp., has been haunting mankind for ages. Unsuccessful attempts to develop a vaccine, the emergence of resistance against the existing drugs and the increasing mortality rate all call for immediate strategies to treat it. Intense attempts are underway to develop potent analogues of the current antimalarials, as well as a search for novel drug targets in the parasite. The indispensability of apicoplast (plastid) to the survival of the parasite has attracted a lot of attention in the recent past. The present review describes the origin and the essentiality of this relict organelle to the parasite. We also show that among the apicoplast specific pathways, the fatty acid biosynthesis system is an attractive target, because its inhibition decimates the parasite swiftly unlike the 'delayed death' phenotype exhibited by the inhibition of the other apicoplast processes. As the enzymes of the fatty acid biosynthesis system are present as discrete entities, unlike those of the host, they are amenable to inhibition without impairing the operation of the host-specific pathway. The present review describes the role of these enzymes, the status of their molecular characterization and the current advancements in the area of developing inhibitors against each of the enzymes of the pathway.

The efficacy of enrofloxacin, alone or combined with metronidazole, in the therapy of canine leishmaniasis

We evaluated the efficacy of enrofloxacin, alone or combined with metronidazole, against Leishmania infantum. The in vitro activity of this fluoroquinolone was assessed using two different methods: a direct test aimed at assessing the drug activity on the parasite, and an indirect test aimed at evaluating the drug effect on macrophage killing, lymphomonocyte activation and nitric oxide production. An in vivo test was also performed on 36 dogs with leishmaniasis, subdivided into three groups, one treated with enrofloxacin, another with enrofloxacin plus metronidazole, and a control group with meglumine antimoniate. The direct test did not show any action of enrofloxacin on the parasite, while the indirect testing showed an enhancement of macrophage killing and an increase in nitric oxide production. These findings show that enrofloxacin does not exert a direct anti-leishmanial activity in vitro. However, on the basis of the positive immunostimulation results shown in vitro and the clinical improvement, particularly of the cutaneous lesions, obtained in several dogs in the in vivo trial, the use of enrofloxacin in association with a specific anti-leishmanial drug can be proposed in the therapeutic protocol of canine leishmaniasis.

In vitro activity of moxifloxacin, levofloxacin, gatifloxacin and linezolid against Mycobacterium tuberculosis

The in vitro activity of moxifloxacin, gatifloxacin, levofloxacin and linezolid was evaluated against 234 strains of Mycobacterium tuberculosis isolated in the Southeast of Spain. All drugs tested showed good activity, with an MIC(90) of less than 1 mg/l, and were active against isociacide and rifampicin resistant strains. Three strains were resistant to isoniazid and to the fluoroquinolones, which suggested the existence of mechanisms of resistance not yet described. These new compounds may prove to be therapeutic alternatives for treatment of multi-resistant tuberculosis and further studies should be done to demonstrate their true usefulness.

In vitro activity of four fluoroquinolones against Mycobacterium tuberculosis

The in vitro activity of ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin against strains of Mycobacterium tuberculosis was studied. Moxifloxacin and levofloxacin showed the greatest activity having an MIC(90) of 1 mg/l. The MIC(90) for ofloxacin was 2 mg/l and for ciprofloxacin 4 mg/l. Further studies should be made to determine the role played by these compounds in the treatment of tuberculosis.

A plastid organelle as a drug target in apicomplexan parasites

Parasites of the phylum Apicomplexa include many important human and veterinary pathogens such as Plasmodium (malaria), Toxoplasma (a leading opportunistic infection associated with AIDS and congenital neurological birth defects), and Eimeria (an economically significant disease of poultry and cattle). Recent studies have identified an unusual organelle in these parasites: a plastid that appears to have been acquired by secondary endosymbiosis of a green alga. Here we show that replication of the apicomplexan plastid (apicoplast) genome in Toxoplasma gondii tachyzoites can be specifically inhibited using ciprofloxacin, and that this inhibition blocks parasite replication. Moreover, parasite death occurs with peculiar kinetics that are identical to those observed after exposure to clindamycin and macrolide antibiotics, which have been proposed to target protein synthesis in the apicoplast. Conversely, clindamycin (and functionally related compounds) immediately inhibits plastid replication upon drug application-the earliest effect so far described for these antibiotics. Our results directly link apicoplast function with parasite survival, validating this intriguing organelle as an effective target for parasiticidal drug design.

Fluoroquinolones in animal health

The fluoroquinolones are a series of synthetic antibacterial agents that are undergoing extensive investigation for both human and veterinary use in the treatment of a variety of bacterial infections. These agents work through the inhibition of DNA gyrase, interfering with the supercoiling of bacterial chromosomal material. As a result, these agents are rapidly bactericidal primarily against gram-negative bacteria, mycoplasma, and some gram-positive bacteria, with most having little to no activity against group D streptococci and obligate anaerobic bacteria. Resistance develops slowly and is almost always chromosomal and not plasmid-mediated. However, development of resistance to the fluoroquinolones and transfer of that resistance among animal and human pathogens have become a hotly debated issue among microbiologists. The fluoroquinolones are a current antimicrobial class whose use in veterinary medicine is being scrutinized. From a pharmacokinetic perspective, these agents are variably but well absorbed from the gastrointestinal tract and almost completely absorbed from parenteral injection sites, and they are well distributed to various tissues in the body. The fluoroquinolones are metabolized and renally excreted, with many of them having approximately equal excretion by the hepatic and the renal excretory systems. The primary toxicity observed at therapeutic doses involves the gastrointestinal system and phototoxicity, although at higher doses central nervous system toxicity and ocular cataracts are observed. Administration to immature animals may result in erosive arthropathies at weight-bearing joints, and administration of high doses to pregnant animals results in maternotoxicity and occasionally embryonic death. The fluoroquinolones are approved for indications such as urinary tract infections and soft tissue infections in dogs and cats and colibacillosis in poultry. Approval for bovine respiratory disease in the United States is being sought. Other indications for which the fluoroquinolones have been used in animal health include deep-seated infections, prostatitis, and other bacterial infections resistant to standard antimicrobial therapy.

In vitro susceptibilities of rapidly growing mycobacteria to newer antimicrobial agents

The in vitro antimicrobial susceptibilities of 42 isolates of rapidly growing mycobacteria (Mycobacterium fortuitum, M. chelonae, and Mycobacterium species [other than M. fortuitum and M. chelonae]) to nine quinolones, including newer agents, two new aminoglycosides, and an aminocyclitol (trospectomycin) were determined by a broth microdilution method. The new quinolones, PD 117596, PD 127391, and PD 117558, showed excellent in vitro activities against M. fortuitum (MICs for 90% of isolates [MIC90s], 0.06, 0.06, and 0.12 microgram/ml, respectively). The MIC90 of ciprofloxacin for M. fortuitum was 0.5 microgram/ml. Only 14 to 28% of isolates of M. chelonae were susceptible to various quinolones. Most isolates of all three species were susceptible to the new aminoglycosides SCH 21420 and SCH 22591. The MIC90s of trospectomycin were 8 micrograms/ml for M. chelonae, 32 micrograms/ml for Mycobacterium species, and > 64 micrograms/ml for M. fortuitum.

Cyclothialidine and its congeners: a new class of DNA gyrase inhibitors

Cyclothialidine is a new, potent DNA gyrase inhibitor isolated from Streptomyces filipinensis. However, it exhibits hardly any growth-inhibitory activity against intact bacterial cells. To explore its potential with regard to the development of a new type of antibacterial, a flexible synthetic route was worked out (i) to investigate the structural requirements for DNA gyrase inhibition and (ii) to search for congeners exerting antibacterial activity. The structure of cyclothialidine was confirmed by total synthesis. Marked DNA gyrase inhibitory activity was found for a number of analogs, a common feature of them being the bicyclic 12-membered lactone bearing one phenolic hydroxy group. Congeners of cyclothialidine were found to exhibit a moderate, but broad-spectrum, in vitro activity against Gram-positive bacteria. Therefore, the DNA gyrase inhibitory principle contained in cyclothialidine can be considered as the basis for a new class of antibacterial agents.

In vitro comparison activity of OPC-17116, a new fluoroquinolone, against more than 5,000 recent clinical isolates from five medical centers

The in vitro activity of OPC-17116, a new fluoroquinolone, was compared with those of ciprofloxacin, ofloxacin and cefixime. A total of 5,231 fresh clinical isolates from five geographically separate US medical centers were tested. The primary results of this study are: 1. OPC-17116 is very active against Enterobacteriaceae [MIC90s, 0.015-1 microgram/ml, except Providencia rettgeri (MIC90, > 8 micrograms/ml)], 2. among nonenteric Gram-negative bacilli (e.g. Xanthomonas maltophilia, MIC90, 2 micrograms/ml) and Gram-positive cocci (e.g. oxacillin-susceptible staphylococci, MIC90s, 0.12-0.5 microgram/ml) OPC-17116 demonstrated higher activity than ciprofloxacin or ofloxacin, 3. the quinolones were more active than cefixime against most of the species tested, especially against Gram-positive and non-enteric Gram-negative organisms. The results of this study, associated with prior documented favorable qualities, support continued investigation of OPC-17116 for clinical use.

In vitro antimicrobial susceptibilities of Nocardia species

The in vitro activities of various quinolones, two new aminoglycosides, a new cephamycin analog (cefmetazole) and a new spectinomycin analog (trospectomycin), imipenem, and trimethoprim-sulfamethoxazole against 26 isolates of Nocardia asteroides, 7 isolates of N. brasiliensis, and 6 isolates of N. caviae were determined by a broth microdilution method. The three new quinolones, PD 117558, PD 117596 and PD 112739, inhibited 90% of N. asteroides at 1 to 2 micrograms/ml, and two new aminoglycosides, SCH 21420 and SCH 22591, inhibited 90% of N. asteroides at 2 to 4 micrograms/ml. Among the beta-lactams, cefmetazole was more active than imipenem. N. brasiliensis and N. caviae isolates were also very susceptible to the three quinolones (MICs for 50% of the isolates, 0.25 to 1 microgram/ml) and the two aminoglycosides (MICs for 50% of the isolates, 1 to 2 micrograms/ml). Cefmetazole was moderately active against N. brasiliensis, whereas imipenem showed poor activity against both of these species.

Impact of the fluoroquinolones on gastrointestinal flora

A number of studies have been performed to evaluate the effect of the fluoroquinolones on gastrointestinal flora. The fluoroquinolones have only slight or no effect on the oropharyngeal flora, except when Neisseria, Haemophilus or Branhamella spp. are present. Studies have consistently shown that Gram-negative facultative bacteria of the lower intestinal flora are strongly suppressed during administration of these agents. Total faecal anaerobes are generally unchanged. The effect of the fluoroquinolones on Gram-positive bacteria is more variable with mild to moderate suppression reported with some agents. In view of the high faecal concentrations of the fluoroquinolones, the general lack of effect on anaerobes is surprising; it may be attributable to the large number of microorganisms found in faeces and faecal binding of the fluoroquinolones. Several recent studies suggest that the effects of some fluoroquinolones on faecal anaerobes and Gram-positive cocci may be more profound in certain patient populations such as bone marrow transplant recipients and patients undergoing gastrointestinal surgery. Colonisation with yeasts and the emergence of resistant bacterial strains have been reported during or after fluoroquinolone administration in some studies. Future studies will need to investigate the effect of the newer agents with greater activity against anaerobes and Gram-positive cocci on the gastrointestinal flora and to continue surveillance for resistant organisms.

The value of new antimicrobial agents

New antimicrobial agents require careful assessment to determine whether they possess the appropriate properties for clinical use. Many new compounds of value in the prophylaxis and therapy of infections have been included in the current armamentarium. Close monitoring to identify possible adverse effects of new agents is essential. Alternative uses of older agents also expands therapeutic opportunities in the combat against infectious diseases. Inappropriate use of antimicrobial agents contributes to the worldwide problem of increasing bacterial resistance. The dramatic spread of such bacterial strains is partially responsible for the rising costs of infectious diseases. A rational and strict antibiotic policy is thus of great importance for the optimal use of these agents.

In vitro activity of the new quinolone lomefloxacin against Mycobacterium tuberculosis

Minimal inhibitory concentrations (MIC) of ciprofloxacin, ofloxacin and lomefloxacin were determined for 90 Mycobacterium tuberculosis strains isolated from both AIDS and other patients. Eleven (12.2%) of these strains showed in vitro resistance to one or more first-line antituberculosis drugs. Susceptibility tests were done in 7H12 broth by the radiometric method. The MIC range for ciprofloxacin was 0.125 to 4.0 micrograms/ml; for ofloxacin, 0.25 to 4.0; and for lomefloxacin 0.5 to 4.0 micrograms/ml. On the basis of our data, we believe that the following MIC, when determined in 7H12 broth radiometrically, should be used as break points to classify the strain as susceptible: ciprofloxacin and ofloxacin, 1 microgram/ml or less; lomefloxacin, 2 micrograms/ml or less. Lomefloxacin on a once-daily basis deserves further evaluation as a potential supplementary drug for the treatment of tuberculosis.

Relative beta-lactamase- and transpeptidase-inhibitory activities of the new quinolone WIN-57273 in Staphylococcus aureus

The new quinolone WIN-57273 was shown to inhibit Staphylococcus aureus beta-lactamase activity noncompetitively in vitro with an apparent Ki value of 0.5 mM. MICs of penicillin G for a highly quinolone-resistant, beta-lactamase-negative strain in the presence of exogenous beta-lactamase decreased considerably when subinhibitory concentrations of WIN-57273 were added. Furthermore, the attachment transpeptidase reaction, investigated on whole cells of S. aureus, was impeded by WIN-57273 concentrations of greater than or equal to 30 microM. While these interactions suggest a novel mechanism of action for this compound, they are probably not relevant to the overall antibacterial potency of WIN-57273.